Enum Property Error: expected a string enum, not tuple
You are attempting to assign a property definition as a property value:

scn.pipe.asset_categories = bpy.props.EnumProperty(...)


Should be: 

scn.pipe.asset_categories = 'new'


Or, if you actually want to change the definition, uou must manipulate the type instead of it's instance:

del bpy.types.Scene.pipe
bpy.types.Scene.pipe = bpy.props.EnumProperty(...)


Where ... are all the enum arguments